(Per)fluoropolyethers (PFPEs) are fluorinated polymers comprising a fully or partially fluorinated polyoxyalkylene chain that contains recurring units having at least one catenary ether bond and at least one fluorocarbon moiety. The most widespreadly known PFPEs can be obtained by means of processes comprising either the homopolymerization of hexafluoropropylene oxide (HFPO) or 2,2,3,3-tetrafluorooxetane or the photooxidation of tetrafluoroethylene (TFE) and/or hexafluoropropylene (HFP).
PFPEs can be divided into non-functional and functional; the former comprise a PFPE chain having at least two ends, wherein such ends bear (per)haloalkyl groups, while the latter comprise a PFPE chain having at least two ends, wherein at least one end comprises a functional group. The most widespreadly known functional PFPEs are mono- and bi-functional PFPEs, i.e they have one or two functional end groups. Functional PFPEs can be used as starting materials for the manufacture of other functional PFPEs that are used as such for a variety of industrial applications, e.g. as additives for lubricant compositions. In addition, functional PFPEs can be used as building blocks for the manufacture of block copolymers.
Certain functional PFPEs wherein at least one chain end bears at least one amine group (herein after otherwise referred to as “PFPE amines”) and methods for their manufacture are known in the art. However, the available manufacturing methods suffer from certain drawbacks.
U.S. Pat. No. 3,810,874 B (MINNESOTA MINING & MFG) 14 May 1974 discloses, in Example XIV, the synthesis of a PFPE diamine of formula:NH2CH2CF2O(CF2CF2O)m(CF2O)nCF2CH2NH2 by reaction of a PFPE disulfonate of formula:CF3SO2CH2CF2O(CF2CF2O)m(CF2O)nCF2CH2SO2CF3 with liquid anhydrous ammonia in a 16:1 molar ratio with respect to the sulfonyl groups of the PFPE. The reaction is carried out under pressure at 100° C. in the absence of solvents. The reported yield is 93%.
However, when the Applicant followed the procedure disclosed in that example, the desired PFPE amine was obtained with a yield lower than 70% and with low selectivity, due to the formation of several by-products. Furthermore, this document does not disclose or give hints to the manufacture of secondary and tertiary PFPE amines, which are also difficult to obtain with high selectivity and yields.
This conclusion was also previously arrived at and reported in TONELLI, Claudio, et al. Linear Perfluoropolyether difunctional oligomers: chemistry, properties and applications. Journal of Fluorine Chemistry. 1999, vol. 95, no. 1-2, p. 51-70. On page 60, par. 2.2.2.4, right hand column, last paragraph, it taught that primary and secondary PFPE amines can be obtained by reaction of a PFPE nonaflate with ammonia or primary amines; however, it is explained that such method is not convenient, unless a large excess of amine or hindered primary amines are used; indeed, since PFPE amines are stronger bases than ammonia, they compete with ammonia or with the primary amine in the reaction with the PFPE nonaflate, thereby leading to the formation on by-products. Furthermore, no teaching or hint is given with regard to any possible effect of temperature on the reaction selectivity.
Examples 1-3 of U.S. Pat. No. 6,984,759 B (SOLVAY SOLEXIS SPA) Oct. 1, 2006 also teach the preparation of a PFPE diamine of formula:NH2CH2CF2O(CF2CF2O)m(CF2O)nCF2CH2NH2.
In this case, the reaction proceeds by catalytic reduction of a PFPE dinitrile of formula:CNCH2CF2O(CF2CF2O)m(CF2O)nCF2CH2CNin the presence of a Pd/Ca F2 catalyst.
This method is, however, quite expensive to be carried out on an industrial scale, in view of the fact that:                the dinitrile must be synthesised by conversion of a PFPE-diester into the corresponding diamide and then by conversion of the diamide into nitrile;        the amount of catalyst is as high as 50% wt with respect to the PFPE-dinitrile.        
Furthermore, the reaction requires the use of a Pd/CaF2 catalyst, having a lower surface area than catalysts based on other metals supported on carbon.
U.S. Pat. No. 5,446,205 B (AUSIMONT SRL) 29 Oct. 1995 discloses, in Example 7, the synthesis of a PFPE amine of formula:Cl3F6O(C3F6)1.27(CF2O)0.05CF2CH2NH2 by reduction of a PFPE amide of formula:Cl3F6O(C3F6)1.27(CF2O)0.05CF2CONH2 with LiAlH4 in ethyl ether as solvent.
Even if this method allows the obtainment of primary, secondary and tertiary amines, it requires the use of a flammable and anhydrous solvent and of LiAlH4 as reducing agent, which is expensive and sensitive to moisture. Furthermore, the workup of the final reaction mixture and the isolation of the PFPE amine is troublesome, in view of the fact that unreacted LiAlH4 is still present and needs to be destroyed.
A method similar to that disclosed in U.S. Pat. No. 5,446,205 is reported in STREPPAROLA, Ezio, et al. Elastomeric polyimides from alpha,omega-Bis(aminomethyl)polyoxyperfluoroalkylenes and Tetracarboxylic Acids. I&C Product Research & Development. 1984, vol. 23, no. 4, p. 600-605.
JP H06219994 (IDEMITSU PETROCHEM CO LTD) Sep. 8, 1994 discloses the synthesis of perfluoroalkylamines of formula RfCH2NH2, wherein Rf is perfluoroalkyl, by reaction of a perfluoroalkylsulfonyl ester with an excess of ammonia under pressure at a temperature ranging from 50° C. to 200° C., usually in the presence of a solvent. The examples specifically teach the synthesis of three perfluoroalkylsulfonyl amines in the presence of DMF as solvent at 100° C. In the examples, ammonia is used in a molar excess of about 2.5 with respect to the perfluoroalkylsulfonyl ester. This patent document does not disclose or provide suggestions to the synthesis of PFPE amines.
EP 0621298 A (AUSIMONT SPA) 26 Oct. 1994 discloses fluorinated polymers having thermoplastic properties comprising in the macromolecule perfluoropolyoxyalkylene sequences of formula: —CF2O(CF2CF2O)m—(CF2O)n—CF2—, wherein m/n=0.2+5.
Such polymers can be obtained by polycondensation of one or more condensation monomers of formula:Z—CF2O(CF2CF2O)m—(CF2O)n—CF2—Z,wherein Z is a reactive group and m and n are as defined above.
Among such condensation monomers, a tosyl sulfonate ester derivative of a PFPE containing ethylene oxide segments is mentioned at entry 5 of Table 1 on page 5. Entry 4 of the same table refers to a monomer wherein —Z is a —CH2NH2 group; despite the fact that entry 4 discloses the synthesis of this derivative by reaction of a monomer wherein —Z is —CH2OSO2CF3 with ammonia, no details on such reaction are provided, nor is the synthesis of PFPE amines from monomers of entry 5 therein disclosed.
EP 0665253 A (AUSIMONT S.P.A.) Feb. 8, 1995 discloses the use of bi- or poly-functional PFPE polymers for the preparation of high-dry formulations for paints and coatings. Examples 1 teaches the manufacture of a PFPE tetraol through a synthetic pathway which starts from the reaction of a PFPE diol with tosylchloride to provide the corresponding sulfonic ester. The resulting disulfonic ester is reacted with ethyl malonate to provide a tetraester which is subsequently reduced to provide the tetraol. This document does disclose the reaction of PFPE sulfonic esters with amines.
EP 1388556 A (SOLVAY SOLEXIS SPA) Feb. 11, 2004 discloses a process for the manufacture of PFPE derivatives, including amine derivatives, by catalytic reduction.
There is therefore the need to provide a convenient method for the manufacture of PFPE amines, in particular primary, secondary and tertiary PFPE amines, that provides high yields and selectivity, avoids the use of expensive and/or excessively dangerous chemicals and that can be conveniently carried out on an industrial scale.